Tantalum oxide (Ta.sub.2 O.sub.5) is often used in the fabrication of optical devices and integrated circuits. For example, tantalum oxide can be used as a mask for etching silicon dioxide (SiO.sub.2) in order to define waveguides in a silicon substrate. In a typical oxide etching operation, fluorine etchants are usually preferred. However, for tantalum oxide, fluorine etchants tend to be fairly slow and have low selectivity to photoresist and SiO.sub.2. Therefore, it is desirable to provide a faster and more selective etching process.